The spot welding process involves: turning on the cooling water; cleaning the surface of the workpieces, assembling them accurately, and then placing them between the upper and lower electrodes, applying pressure to ensure good contact; applying current to heat the contact surfaces of the two workpieces, causing localized melting and forming a weld nugget; maintaining pressure after power is cut off to allow the weld nugget to cool and solidify under pressure, forming a weld spot; removing the pressure and removing the workpiece. Spot welding process parameters such as welding current, electrode pressure, energizing time, and electrode working surface dimensions have a significant impact on welding quality.
A spot welding machine utilizes the high-temperature arc generated when the positive and negative electrodes are momentarily short-circuited to melt the materials to be welded between the electrodes, achieving the purpose of bonding them. The structure of a spot welding machine is very simple; essentially, it's a high-power transformer that converts 220V AC power into a low-voltage, high-current power supply, which can be either DC or AC. Welding transformers have a unique characteristic: a rapid voltage drop.
After the welding rod is ignited, the voltage drops. The working voltage of the welding machine is adjusted not only through the primary 220/380V voltage conversion, but also through taps on the secondary coil. Adjustable iron cores are also available. A welding machine is generally a high-power transformer, built using the principle of inductance. The inductance generates a huge voltage change when switched on and off. The high-voltage arc generated by the instantaneous short circuit between the positive and negative poles melts the welding material on the welding rod, achieving the purpose of joining them.
Spot welding is a resistance welding method where workpieces are assembled and pressed between two electrodes. Resistance heat is used to melt the base metal, forming a weld point. Spot welding is often used for joining thin plates, such as aircraft skin, aircraft engine smokestacks, and automobile cab shells. The welding transformer in a spot welding machine is the spot welding electrical appliance; its secondary winding has only one loop. The upper and lower electrodes and electrode arms are used to conduct welding current and transmit power. Cooling water should pass through the transformer, electrodes, etc., to prevent overheating during welding. Cooling water should be supplied first, then the power switch should be turned on. The quality of the electrodes directly affects the welding process, welding quality, and productivity. Electrode materials commonly used include copper, cadmium bronze, and chromium bronze; electrode shapes vary, mainly determined by the shape of the workpiece. When installing electrodes, ensure the upper and lower electrode surfaces are parallel; the electrode surface should be kept clean, often using sandpaper or a file for finishing. The welding cycle for spot welding and projection welding consists of four basic stages (spot welding process):
(1) Pre-pressure stage-The electrode descends to the current-on stage, ensuring the electrode presses firmly against the workpiece, creating appropriate pressure between the workpieces.
(2) Welding time-The welding current passes through the workpiece, generating heat to form a weld nugget.
(3) Holding time-The welding current is cut off, and the electrode pressure continues to be maintained until the weld nugget solidifies to sufficient strength.
(4) Rest time-From the moment the electrode begins to rise until it begins to descend again, the next welding cycle begins.
To improve the performance of welded joints, it is sometimes necessary to add one or more of the following to the basic cycle:
(1) Increase preload to eliminate gaps between thick workpieces, ensuring a tight fit.
(2) Use preheating pulses to improve the plasticity of the metal, making it easier for the workpieces to fit tightly and preventing spatter; in projection welding, this ensures that multiple projections are in uniform contact with the plate before welding, guaranteeing consistent heating at each point.